1. Field of the Invention
The present invention pertains to a position detecting device, and more particularly to a magnetic encoder which can write an encoded signal and then read the same encoded signal to determine a position of a moving object.
2. Description of the Related Art
Several techniques for monitoring position of a moving object, such as a rotating shaft, are known. Typically, an encoder is employed to determine the position of a rotating shaft. Once the position is determined, information such as speed and rotational direction of the rotating shaft can be determined by recording the change in position over time. Such encoders may determine either "incremental" or "absolute" positions and may be implemented by various means, including magnetic means and optical means.
Referring to FIGS. 1 and 2, a typical 8 bit disk 20 useful with an absolute encoder and its respective output 30 is shown. An absolute encoder provides a "whole word" output with a unique code pattern representing each position. The code is derived from light transmitting sources 32, such as light emitting diodes (LEDs), passing through slits 33 which are placed on independent tracks corresponding to individual photodetectors 34. The output from these detectors would be high or low ("1" or "0" respectively) depending on the code disk pattern for that particular position. For example, disk pattern 36 sensed by photodetectors 34 yields an output having a digital binary value of "1". The number of tracks determines the amount of information that can be derived from a revolution of the disk. For example, if the disk has ten tracks, the resolution of the encoder would be 2.sup.10 or 1024 positions per revolution of the disk. In order to count multiple revolutions additional disks are attached to the primary high resolution disk and geared at some ration, for example, an 8:1 ratio. There is no theoretical limit to the number of disks that may be added; however, there are practical limits and limitations with space have resulted in encoders that typically have no more than 512 turns available.
An absolute encoder determines the actual position of a rotating shaft at any point of the shaft's revolution within an established radial resolution and within a fixed number of total revolutions. Absolute encoders rely on determining that a fixed location on the disk (e.g., the location of an LED) is presently at a predetermined position. Thus, they are not well suited for monitoring swiftly changing conditions. Typical applications are those where a device is inactive for long periods of time or moves at a slow rate, such as flood gate control, telescopes, and construction cranes.
An incremental encoder determines the position of a shaft by providing markers at discrete measured intervals which can be counted to determine how far the shaft has rotated. This is commonly accomplished by attaching an incremental encoder directly to the end of a motor shaft. The motor is controlled in a conventional manner to rotate the shaft to a certain position or turn at a designated speed. The encoder responds by creating a square wave and the peaks of the wave can be counted by known circuits to determine the position of the rotating shaft. Once the one position of the shaft is known, and the time between one position to the next is known, the speed and direction (velocity) of the motor can be determined. By using a known technique of closed loop feedback control, a precise desired speed of rotation can be achieved after some passage of time. Another common type of incremental encoder allows for feedback control by measuring relative speed or angle of rotation and comparing the measured values to known values such as clock time, or angular position relative to a known fixed point.
Referring to FIGS. 3 and 4, an incremental code disk 40 typically employed with an optical incremental encoder 50 is shown. A series of slots 42 in the disk 40 are counted as the disk rotates in the following fashion. In high resolution encoders the spacing 44 between slots can be as small as 50 microns. A light source 60 mounted on bearing housing assembly 62 shines a beam of light continuously. As the code disk rotates on spindle 64 the light shines through the slots 42. For increased light resolution the light source is collimated and passed through a mask 58. The mask is disposed between the disk and the photodetector 56 so that the slots 42 and the mask 58 produce a shuttering effect. As a result, light is only allowed to pass to the photodetector 56 when the slots and the transparent sections of the mask are in alignment. The photodetector 56 is part of the photodetector assembly 57. The photodetector assembly is coupled to the electronics board 54 such that a signal can be created in response to the sensing of the light through the mask. The counter circuit 52 in electronics board 54 counts the pulses so that the number of slots having rotated past the photodetector is known. Since each slot corresponds to some incremental angular displacement, determining the number of slots also determines the total angular displacement of the disk.
The following example explains the operation of an incremental encoder. A typical prior art incremental encoder creates a series of square waves. The number of square waves corresponds to a predetermined mechanical increment. For example, to divide a shaft revolution into 1000 parts, an encoder could be selected to supply 1000 square wave cycles per revolution. By using a counter circuit to count those cycles, it could be determined how far the shaft has rotated. For example, 100 counts would equal 36 degrees, and 1000 counts would equate to an entire revolution. Unfortunately, the number of cycles per revolution is limited by the amount of slots that can be placed in the disk. Thus, physical parameters such as physical line spacing, also known as granularity, and quality of light transmission limit the precise measurement of rotational displacement. Consider the example of a disk that is divided into 1000 parts and another that is divided into 10000 parts. Naturally, the one having 10000 parts has 10 times more granularity and this allows for precise measurements. Unfortunately, there are practical physical limits that prevent achieving such fine granularity on an optical disk. The current technology permits incremental resolution up to 2540 cycles per turn on an encoder disk. Higher resolutions are available through various multiplication techniques but an inherent disadvantage is the extra time and performance overhead associated with such cumbersome calculations. Generally, incremental encoders provide more resolution at a lower cost than their absolute encoder cousins.
Magnetic encoders are generally not preferred over optical encoders because they are very large by comparison. Also magnetic encoders have upper limits of around 1200 discrete pulses per revolution. Typically, magnetic encoders operate under the principal of a heavy duty rotating magnetized dram placed in proximity to a read element such as a non-contact magnetoresistive (MR) sensor. Such a magnetic encoder is shown in Japanese laid-open patent applications, application numbers 62-39178 and 62-39179. In such an encoder, a drum rotates as a MR sensor picks up the changes in the magnetic fields and produces a square wave that is translated to the high/low or digital "1"/"0" states. Accordingly, the position of a shaft coupled with the magnetic encoder can be determined by the amplitude readings from the square wave.
A variation of a magnetic encoder useful as an absolute encoder is disclosed in U.S. Pat. No. 4,599,561 by Takahashi et al. The disclosed encoder employs a plurality of MR sensors formed opposite to an encoded track on a rotating body. The encoded track that is used for speed control is disclosed as being arranged to be magnetically opposed to a position detecting track in order to eliminate magnetic interference. The position detecting track is used in the conventional sense to determine the rotational position of a rotating body. The encoded track contains information on how to respond to this position information. Unfortunately, Takahashi does not disclose any means for changing the encoded information or the position information once the encoder is created. That is the encoder can not be changed dynamically to suit a new purpose.
In general all of the above listed devices, optical and magnetic, have an electronic data transfer rate in the 100-200 Khz range. Although some optical incremental encoders cost as little as $200 to $500, the more sophisticated absolute encoders tend to cost around $2000. The very high encoders having the highest resolution may cost around $5000 to $6000. Generally, cost increases as resolution increases.
Another disadvantage of prior art optical encoders is that an encoder being configured to have a specified resolution can not be changed to another resolution. This is also the case with magnetic encoders once the track is encoded with magnetic information. It would be desirable to provide a means wherein a low cost but high resolution encoder could be changed dynamically according to the need at any given time. However, prior to the present invention there has not existed such an encoder.